1. Field of the Invention
The present invention relates to an acceleration sensor.
2. Description of the Related Art
Japanese Patent No. 2780594 discloses an acceleration sensor employing a piezoelectric ceramic. This acceleration sensor includes a bimorph type detection element which is produced by laminating a pair piezoelectric devices formed of piezoelectric ceramics into a unitary body. The detection element is supported at both ends thereof and housed in a casing. When acceleration acts on the acceleration sensor, the detection element is deflected, and stress is generated in the defection element. The acceleration sensor detects acceleration, by detecting charge or voltage generated through piezoelectricity. The acceleration sensor is compact enough to be easily structured into a surface-mounting component (a chip component).
In the acceleration sensor working on this principle, a bias current flowing from a circuit of the sensor into the piezoelectric body is charged in a capacitor C of the piezoelectric body, and a resistor R is thus required to leak the bias current. The resistor R and the capacitor C form a filter, thereby leaving a direct current and a low frequency component below a cutoff frequency thereof undetected.
In a known acceleration sensor (disclosed in Japanese Unexamined Patent Application Publication No. 4-361165), two vibrators are mounted on a flex-type tuning fork support body. When acceleration acts on the vibrators, the vibrators mounted the tuning fork support structure are subject to tensile stress and compressive stress at central inertia portions (weight portions), and acceleration is detected from a frequency difference taking place between the two vibrators. This acceleration sensor detects a direct-current and low-frequency components.
Since the acceleration sensor thus constructed has the support body of tuning fork, the design of the structure becomes complex and bulky, and extensions of electrodes from the vibrators are also complex. It is therefore difficult to arrange this acceleration sensor in a miniature surface-mounting component (a chip component) which may directly be mounted on a printed circuit board.
The tuning fork vibrator is designed as a bimodal tuning fork vibrator to vibrate in a combination vibration mode in which a torsional vibration mode and a flexural vibration mode are combined, thereby reducing dependency of a bias frequency on temperature. This arrangement fails to fully eliminate the temperature dependency thereof.
Accordingly, it is an object of the present invention to provide a compact and high-gain acceleration sensor which is surface-mounted and insensitive to factors such as temperature changes other than acceleration.
The above object is achieved by the present invention as will be discussed below.
The present invention in one aspect includes a bimorph type acceleration detection element including a first resonator, a second resonator and an intermediate layer, in which the first resonator and the second resonator are bonded with the intermediate layer interposed therebetween. Each resonator includes a piezoelectric body and electrodes arranged on both main surfaces thereof. The intermediate layer is hard enough to transmit flexural stress in one of the first and second resonator to the other of the first and second resonators and the vibration of the one of the first and second resonators is attenuated to be transmitted to the other of the first and second resonators. The acceleration detection element is supported at one longitudinal end or both longitudinal ends thereof such that the first and second resonators are deflected in the same direction according to the application of acceleration. Acceleration is detected by detecting a difference between frequency changes of the first resonator and the second resonator or a difference between impedance changes of the first resonator and the second resonator which is caused by deflection of said acceleration detection element.
The acceleration sensor of the present invention has a bimorph structure in which the acceleration detection element is produced by bonding the two resonators together with the intermediate layer interposed therebetween. The intermediate layer has a hardness that allows flexural stress to be transmitted from one resonator to the other resonator. When acceleration is applied, the acceleration detection element deflects and is distorted, then tensile stress acts on the one resonator, and compressive stress acts the other resonator. The intermediate layer has the function of modestly mechanically coupling the two resonators in vibration transfer. In other words, the vibration of the one resonator are attenuated to be transferred to the other resonator. For this reason, each resonator vibrates at its own natural frequency. The frequency of the resonator on the tensile side of the element becomes lower, while the frequency of the resonator on the compressive side of the element becomes higher. Acceleration is thus detected by differentially picking up a difference between frequency changes of the two resonators or a difference between impedance changes of the two resonators.
Since the frequency difference or the impedance difference is detected, rather than individually picking up the frequency changes of the two resonators or the impedance changes of the two resonator, stresses commonly acting on the two resonators (a stress due to a temperature change, for example) cancel each other out. A high-gain acceleration sensor free from the effect of temperature changes is provided.
Preferably, the intermediate layer is constituted of an elastic adhesive layer. The intermediate layer transfer flexural stress and the vibration of the one resonator is attenuated to be transmitted to the other resonator. Using the elastic adhesive layer, these functions are easily performed.
An epoxy-based adhesive agent, an epoxy-acrylic adhesive agent, or silicone-based adhesive agent may be used for the elastic adhesive layer. When the epoxy-based adhesive agent or the epoxy-acrylic agent is used, the thickness thereof may be as thick as several xcexcm to dozens of xcexcm. When the silicone-based adhesive is used, the thickness thereof is as thick as several xcexcm because of its small elastic coefficient.
Preferably, each of the first and second resonators is a vibration mode element in which energy is trapped to the longitudinal center portion thereof, and a gap is provided in the longitudinal center of the intermediate layer. The gap is larger in area than a trapped vibration region of each of the first and second resonators and smaller in area than a deflection region of each of the first and second resonators which deflect according to acceleration.
Laminating the resonator and the intermediate layer on the entire surfaces thereof is acceptable. However, it is noted that such an arrangement reduces performance of the resonator (such as Q and K factors) because the vibration of the resonator is restricted by the intermediate layer. Since the intermediate layer has, in the longitudinal center thereof, the gap which is larger in area than a trapped vibration region of each resonator and smaller in range than a deflection region of each resonator which deflects under acceleration, the transfer of the vibration is controlled even if the non-elastic material is used as the intermediate layer. Furthermore, the intermediate layer transfers flexural stress from one to the other resonator.
Preferably, the external surface of the acceleration detection element, facing in a direction in which acceleration is applied, is covered with a casing member, and each open surface which the acceleration detection element and the casing member define is covered with a covering member, and the electrodes arranged on the first and second resonators are respectively connected to external electrodes arranged on the surface of the covering member via internal electrodes arranged on the surface of the casing member.
The acceleration detection element is fully enclosed in the casing member and the covering member, and is thereby appropriate for use as a surface-mounting electronic component.
The acceleration sensor of the present invention uses two methods for differentially picking up the signals from the first resonator and the second resonator and for obtaining a signal proportional to acceleration acting on the acceleration detection element. In one method, the first and second resonators are oscillated at different frequencies, a difference between the oscillated frequencies is detected, and a signal proportional to acceleration is determined from the frequency difference. In the other method, the first and second resonators are oscillated at the same frequency, one of a phase difference and an amplitude difference is obtained from a difference between electric impedances of the two resonators, and a signal proportional to acceleration is determined from one of the phase difference and the amplitude difference.
Acceleration is detected with high accuracy using either of the above two methods.